Carrier telegraph system



May 25, 1937. v, R THQRP 2,081,312

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V. P. THORP CARRIER TELEGRAPH SYSTM Filed June 5 1934 2' sheets-smet4 2mvENToR BY 7j? Tho/fp ATTORNEY article 'l a 'reine ysrsrr WaughnP.Thorp, River Edge, N. il., assigner to i r rican Telephone and|ii'elegraph Company,

" corporation ci New Yorh application rune s, wel, serial Nc. vzaisv 6@la/ima.

This invention relates to carrier telegraph systems and,moreparticularly. to arrangements for reducing the distortion of thetelegraph'signals received over such systems.

In carrier telegraph systems ofthe usual type any changes in thereceived currents are apt to cause distortion in the telegraph signals.These changes in received current are due to a number oi causes andprimarily tovariations in line attenuation with weather conditions. Itis the practice in certain types of carrier telegraph sys--V tems toutilize sensitive receiving relays, which are biased to spacing by acurrent which has a magnetic edect on the armature of the relay of one'half that-of the marking orreceived rectified current. This Vbiasingcurrent is substantially constant and is independent of the linecharacteristics since it is supplied from a local battery. It isevident, therefore, that any change in the received current from thedistant endwill cause bias or distortion in the telegraph signals inthereceiving loop circuit. In certain types of service, such as high speedteletypewriter service, this may be sumciently large to cause trouble.Accordingly, it is one of the primary objects of the invention toprovide arrangements whereby variations in the received currents due tochanges in line attenuation or to other causes will not cause bias ordistortion in the signals in the receiving loop circuit.

While this vinvention will be pointed out/with.

particularity in the appended claims, the invention itself, both as toits further objects and ieatuies, will be better understood from the devtailed description hereinafter following, when read in connection withthe accompanying draw- .1 in which Figure 1 shows an embodiment of theinvention given merely for the purpose of illustration, this gureshowing the sending and signals may be transmitted to line L1 and abranch line Ls over which carrier telegraph'si'gnals may be receivedfrom line Li. Line L1 in this particular cme is aV two-wire line anddifferent frequencies are used for sending to and receiving from thedistant station. The same frequency can bev and (Ci. litt-66) often isused for transmission and reception but in the latter case a 4-wirecircuit is used, the two pairs being connected respectively to L2 and L3through suitable transformers. This will be apparentto those skilled -inthis art.

For purposes of illustration of the invention Vthe local loop .circuitis 'shown full-duplex, that is, separate loops are used for transmissionand reception of signals. Half duplex arrangements may be used equallywell in the practice of this invention but these are not shown on thedrawings since that is not necessary for an understanding of theinvention. For transmission of the telegraph signals, the well knownbalanced loop is used for actuating the sending relay. This will not. bedescribed in detail as it does not constitute an important part of theinvention. I am concerned here particularly in any arrangement' foractuating the sending relay by. means of the operation of thesubscribers sending` key K1.

When key K1 is closed,A the armature of the sending relay will restagainst its M contact and the carrier current from the carrier supplyWill flow `ireely into the sending illter F1, branch line L2, throughtransformer T3 to line Li and to the distant station. When key K1 isopened, the armature of the sending relay will move to its S contact,which connects resistance R1 in shunt with the carrier supply O and thiswill reduce the input to the sending filter F1 by a predetermined amountbut not to zero. When resistance R1 shunts 'the carrier supply O, asubstantial current will still be supplied to the filter F1. Otherarrangements may be used for accomplishing this purpose, and these willoccur to those skilled in this art, but it will be necessary always tokeep in mind that the fundamental method of transmitting signals inaccordance with this invention is by means of varying the transmittedcurrent over A a relatively small range rather than by changing Fig. 1.Carrier signals are received from the distant station over line L1, thenpass hrough transformer T3 to line L3, where theyA are selected fromother signals of other frequencies by the receiving lter F2 and passinto the amplifler A and from there g'o through transformer T1 to thedetector tube D. The outputof the detector tube D includes one Winding,i. e., the primary winding of the transformer T2, the other winding, i.e., the secondary winding of which connects to the operating windings ofthe polar receiving relay. This relay is provided with holding windingsH which serve to hold the armature on whichever contact it happens to beresting when there is no current in the operating windings. It isevident that, when either a steady carrier current or zero current isapplied to the input to the detector, there will be no current in theoperating winding of lthe receiving relay since the direct currentcomponent is kept out of the operating windings of this relay by thetransformer T2. However, when a change of current occurs in the platecircuit of the detector tube, a current will be induced in the secondaryof T2 which will ow in the operating windings-of the receiving relay andits magnitude will be such as to overpower the holding current, thusmoving the armature of the relay to the oppositemcontact.- The windingsoi.' the relay are so poled that an increase of current in the detectorplate circuit will move the relay armature to its M contact and adecrease in detector plate current will move the relay armature to its Scontact. Movement of the receiving relay armature from one contact tothe other connects alternately positive and negative batteries B4 and B5to the receiving loop L5 which actuates the subscribers receivingmechanism Sz.

It will be noted that the polar receiving relay is arranged to operateon changes in received carrier current. There is no local bias currentsupply for this relay, although there is a holding circuit to keep therelay armature on the contact to which it has been moved by the pulse ofcurrent, either marking or spacing from the transformer Ta in thedetector output circuit. 'Ihe holding circuit is unbiased regardless ofinequality in the receiving leg battery voltages and this is because thereturn circuit from the relay armature is brought to the center tap of aresistance Z connected across batteries B4 andl B5, as shown.

A further feature of the invention is the arrangement of the detectorinput and output circuits so that the detector characteristic will havea large range over which the output has a linear relation tothe input..'I'he direct current resistance of both the input and outputof thedetector is kept as low as possible for this purpose. This improvementis indicated more clearly upon a consideration of curve B especially ifcompared with curve C in Fig. 2 which is a normal characteristic for adetector circuit.v

The foregoing explained the operation of the circuits for sending andreceiving signals by means ofy small changes in the transmitted andreceived currents. For an understanding oi' the performance of such asystem yunder various line equivalent conditions, reference is made toFig. 2 of which curve B shows the detector plate current plotted againstdetector input level. Assume, for example, that thenormal equivalent ofthe line is such that the input level to the detector for a marlnngsignal is Xml and for a spacing signal is Xsi. The correspondingdetector plate currents will be IMi and ISI. The change in input levelvfrom marking to' spacing is independent of line conditions since it isdetermined by the resistance R1 and the position of the sending relayarmature. This charge in input level is indicated in Fig. 2 by C1. Thechange in detector plate current for this level is from IM1 to IS1. orvice versa,` and this change in current through the primary of T1 inFig. 1 induces the operating current in the receiving relay windings asshown by curve S in Fig. 3. If the line equivondary winding oftransformer Tn.

alent' changes so that the input level to the detector fora markingsignal is now Xmz and for a spacing signal X82, the change in detectorplate current will be IMa-ISz when going from the marking to the spacingcondition. This change is substantially equal to IMi-IMz so that theeiect on the receiving relay is the same as for the normal level case.The same conditions optain when the line level drops to such a valuethat the input level for aymarking signal is Xm: and for a spacingsignal Xss. It is thus evident that over a considerable range of lineequivalents, the telegraph signals will be received free from bias whichwould ordinarily be present in the ordinary sending and receiving means.

In addition to compensating for line attenuation changes, the circuitarrangements of this invention take care to some extent of the change inbattery voltages at the terminal stations. At

4the sending station, for example, the oscillator and sending'amplierwhich together supply they carrier current will vary in output withcharges in battery potentials and at the receiving station the gain ofthe receiving amplifier will likewise vary with changes in batterypotentials. In any case involving the features of this invention, thepulse of current which operates the relay to its M contact will be thesame in magnitude as that which operates it to its S contact and signalswill be repeated unbiased.

In Fig. 3, the two upper curves `are practically the same. They showthat the plate current ilowing through the primary winding of thetransformer Ta is independent of large changes in, for example, the lineequivalent of the transmission circuit. Curve S shows the current in thesec- 'Ihe square cornered curve H shows the holding current ilowing.through the holding windings H of the receiving relay. 'I'he lowermostcurve shows the current in the receiving loop Ls.

.While the invention has been disclosed as embodied in certain specicarrangements which are deemed desirable, it is'understood that it iscapable of embodiment in many and other widely varied forms withoutdeparting from the spirit oi.' the invention as defined by the appendedclaims.-

What is claimed is:

1. The method oi' transmitting carrier telegraph signals which consistsin continuously and uninterruptedly transmitting a carrier currentduring signaling, changing the magnitude of the carrier current duringits transmission by a small predetermined amount in accordance withmarking and spacing signals and maintaining the said changes inmagnitude of the said carrier current at the transmitting and receivingterminals substantially free of bias independently of variationsigllzetransmitting4 medium within predetermined 2. In a carrier telegraphsystem, a source of carrier current which is uninterruptedly emitted forconveying signals to a distant point, means for varying the magnitude ofthe uninterrupted current emitted by said source by' a small,predetermined amount corresponding to telegraph signals, and means formaintaining the said changes .in magnitude of the said carrier currentat the transmitting and receiving terminals substantially free of biasindependentlyof variations in the transmitting medium withinpredetermined umits. l I 12' s 3. In a lcarrier telegraphsystem, themethod of reducing the eitect of line equivalent changes which consistsin transmitting uninterruptediya carrier current the amplitude of whichvaries by Va predetermined amount inaccordance with signais andmaintaining in the uninterrupted car- `rier current a xed. amplitudechange during reception independent of variations in the conditions oftransmission.

4. The method of signaling which consists in generating a carriercurrent, varying the magnitude of said carrier current by a small amountin accordance with telegraph signals without interrupting said carriercurrent, transmitting said modulated carrier current, receiving saidmodulated carrier current, rectiiying it into current pulsescorresponding to the original signals, and maintaining said pulses cf amagnitude which is always independent of the medium of transmission.

5. The combination of 'a source of alternating current of largeamplitude, means for tranmittina said alternating current and varyingits magnitude by a small amount in accordance with signais, thetransmitted signal bearing current always being ci substantialmagnitude, a vacuum tube detector upon which said alternating curvrentis impressed, and a. circuit coupled to said detector to which thedetected current is transmitted, said detector having input and outputcircuits oi' low resistance so that the characteristic of the detectorwill have a large range over which the output circuit current bears alinear relation to the input circuit current. A v 6. In acarrier'telegraph system, a line havin a transmission characteristicwhich is highly variable-and transmitting current of large but slightlyvariable amplitude corresponding totelegraph signals, a'detector havinglow resistance input and output circuits, the input circuit oi saiddetector being coupled to said line, a circuit coupled to the outputcircuit of said detector in which pulses oi detected current ow. andmeans including said detector and the circuit coupled to the outputcircuit of the detector whereby said pulses will maintain an amplitudevariation in said cir cuit coupled tothe output circuit which is alwaysindependent of the variations in the transmission characteristic oi'said line.

VAUGHN P. THQRP.

